I was using a mobile browser before, and the page layout was different, putting the big summary box full of photos (which excerpts and links to Wikipedia) at the top.
On a desktop, I see what you mean; UrbanDictionary is indeed at the top of the plain-text search results.
Not going to defend Google at all. But for myself, if I'm looking for a Wikipedia-grade introduction to some topic, I usually just search Wikipedia directly. Wikipedia has become so ridiculously useful that it is the top result of a Google search half the time anyway. Wikipedia is a better human-curated web index than any of the old-school 1990s human-curated web indexes ever were.
So maybe it's based on location? Ranked higher based on browser, IP, or something else?